Polyamide resins are generally widely used as engineering plastics having excellent mechanical properties, chemical resistance, oil resistance, gas barrier properties and the like. Xylylenediamine-based polyamide resins obtained by polymerizing xylylenediamine such as m-xylylenediamine and an aliphatic dicarboxylic acid such as adipic acid (hereinafter sometimes referred to as “polyamides XD”) are widely used because they have higher strength, higher elastic modulus, lower water absorption and better gas barrier properties as compared with polyamide 6 and polyamide 66 and the like and they can be further coextruded or coinjection-molded with thermoplastic resins such as polyethylene terephthalate, polyamide 6, polyethylene and polypropylene. For example, oriented multilayer films prepared by orienting a coextruded laminate of polyamide 6 and polyamide MXD6 made from m-xylylenediamine and adipic acid or multilayer sheets comprising a coextruded laminate of polypropylene and polyamide MXD6 have been commercialized as food packaging materials.
However, any unstretched thin single layer film could not be constantly and continuously prepared from polyamides XD because of their low elongation, high flexural modulus and brittleness so that if one tried to prepare a single layer film having a thickness of 50 μm or less, especially 30 μm or less, the resulting film would be easily broken by stress or tension applied to the film during molding even though polyamides XD could be readily molded into single layer films having a thickness of 100 μm or more. Even if they could be constantly prepared into thick films, wrinkles occurred in the films to severely impair their commercial value due to the frictional resistance between the film surfaces when they were taken up in rolls.
Further, it would be desirable to laminate a thin single layer polyamide XD film having a thickness of 30 μm or less with a polypropylene film or the like to form a highly retort-resistant packaging material or to laminate a thin film with a carbon fiber or glass fiber or plant fiber and hot-press the laminate to form a molded article. However, any single layer polyamide XD film that can be applied to even such requirements has not been provided because any roll of a thin single layer film could not be constantly supplied due to the problems described above.
On the other hand, a method for reducing adhesion between films or friction between a film and another member or the like has been known by adding inorganic particles to form protrusions on the surface of the film for the purpose of improving stability during molding of the film (see patent document 1). However, if one tried to apply this method to polyamides XD, the incorporation of inorganic particles would further reduce the elongation of the polyamides XDs and rather increase breakage during the preparation of a film, thus failing to solve the problem that a single layer polyamide XD film cannot be constantly produced in the form of a roll, especially that it is very difficult to produce a thin single layer film of 30 μm or less. Moreover, polyamides XD have the problem of difficulty in uniformly dispersing inorganic particles therein. Additionally, another problem resides in that this method cannot be applied to purposes in which inorganic particles cannot be incorporated.